Color printing



' June 5, 1961 G. 5. J. ALLEN El'AL 2,987,571

' COLOR PRINTING Filed March 3. 1958 3 Sheets-Sheet 1 By 2. M

A Home y June 6, 1961 G. s. J. ALLEN ETAL COLOR PRINTING 3 Sheets-Sheet2 Filed March 3, 1958 vlll.

June 6, 1961 G. s. J. ALLEN EI'AL 2,987,571

COLOR PRINTING Filed March 3, 1958 3 Sheets-Sheet 5 Inventors Mm I. 77M

MAM. W QM A ftorney United States Patent 2,987,571 COLOR PRINTING GordonStanley James Allen, and David Harry Mawby,

London, England, assignors to J. F. Crosfield Limited,

London, England, a company of Great Britain Filed Mar. 3, 1958, Ser. No.718,748 Claims priority, application Great Britain Mar. 11, 1957 16Claims. (Cl. 178 5.2)

This invention relates to the reproduction of colored originals.

In color printing, a reproduction of a color original is made by thesuperposition of differently colored inks, the amount and distributionof each ink being determined by photographing the original throughfilters of suitable colors. Thus in three'color printing, photographs ofthe original are made through red, green and blue filters, and theresulting negatives are used to obtain positives from which the printingcylinders or plates are prepared. By means of the three printingcylinders, cyan magenta and yellow inks are laid down, usually on whitepaper, to absorb red, green and blue light, respectively, where thesecolors are not required. However, in addition to non-linearitiesintroduced by the photographic system and by any etching systememployed, the printing inks themselves suffer from impurities. As anexample, cyan ink appears to contain a large amount of magenta, that isto say, the cyan ink absorbs some of the green light, which it shouldreflect completely.

To overcome this difliculty, a correction which can be calledsingle-stage linear masking, can be introduced photographically. Thecorrection required is the reduction of the density of each point on themagenta printer (taking the example given above) by an amount whichvaries with the density of the corresponding point on the cyan printer.To achieve this correction, a positive mask with a reduced contrastrange is made from the cyan printer negative, and if the mask is thensuperimposed in register on the magenta printer negative, an exposurethrough the combination produces a magenta printer positive which is atleast partly corrected.

One disadvantage of this system is that, since all the negatives havedensities which are substantially equal to each other in areasrepresenting grey tones, the density contrast of the grey tones obtainedby the combination of the negative being corrected and the positivecorrecting mask is less than the contrast of the tones in the negativealone. This reduction must not be too great if the required contrast isto be obtained in the subsequent positive.

The greatest drawback of single-stage masking, however, is its inabilityto overcome the non-linearity known as additivity failure. When printinginks of different colors are to be superimposed, the sum of theireffective densities measured separately is often considerably higherthan the density of the superimposed inks. It has been stated above thatcyan ink appears to contain a large amount of magenta. If a given weightof cyan ink is deposited on two areas, one containing very littlemagenta ink and the other containing a large amount of magenta ink, theetfect of the apparent magenta in the cyan ink may be very great in theformer area, but quite small in the latter area, which is already almostsaturated with magenta. If, as in single-stage linear masking, thedensities of both areas on the corresponding black-and-white magentaprinter positive are reduced to the same extent, the area containing alarge amount of magenta ink will be seriously overcorrected, which isvery undesirable in color reproduction.

' It has been proposed to overcome these drawbacks to some extent by aphotographic process which may be ice called two-stage masking. Intwo-stage masking the correcting mask for a given negative is made froma combination of the negative of the correcting color and a positivemade from the negative to be corrected. Thus, the positive made from themagenta negative is of such a range that when it is registered with thecyan printer negative the neutral scale of densities is cancelled out,that is to say the scale for which the densities for magenta and cyanare equal. Thus there is no reduction of the contrast of the neutralscale. However, considerable non-linearity must be introduced into theprocess if good correction is to be obtained in colors other thanneutral tones. In an electro-optical scanner of the kind envisaged inthe present invention, a large range of modification of the correctingsignal would be required accurately to compensate for thisnon-linearity.

There is, however, a limit to the amount of compensating non-linearitywhich can be introduced into an equipment designed to provide aconsistent performance and to be capable of reproduction with the sameperformance, and moreover it is found that when such extremenonlinearity is provided any small variations in densities or in thecorresponding density signals from one subject to another may cause aconsiderable change in the resulting print, and small blemishes in theoriginal may be greatly magnified. As an alternative, the non-linearitymay be set so that the system will give satisfactory, although notperfect, color reproduction, but so that the results obtained from itwill be consistent from one subject to another and from one day toanother. If it is accepted that perfect color reproduction cannot beachieved, it is important to appreciate where the compromise should bemade. As an example, it is usually more serious for a printer toovercorrect than to undercorrect.

In the method of color reproduction according to the invention anoriginal or uncorrected transparencies or prints derived from theoriginal are scanned to provide electric signals representing colorinformation, and an electric signal representing a correction to beapplied for a given color is obtained by deriving, from a signalrepresenting the color to be corrected and a signal or signalsrepresenting a correcting color or colors, a resultant signal whichrepresents the result of subtracting the density or a function thereofof one or more correcting colors from the density or a function of thedensity of the color to be corrected. The resultant signal is passedthrough a non-linear circuit which is arranged to attenuate the signalwhen it exceeds a predetermined amount. The output of the non-linearcircuit is the correcting signal, and a photographic emulsion serving toprovide the printer for the given color is then exposed in acordancewith the density of the transparency to be corrected as modified by thesaid correcting signal.

Although the combining of the signals representing the color to becorrected and the correcting color (if a single correcting color isassumed) must be carried out in such a manner that the color densitiesare added in opposing sense (for example, in opposite polarities), thesignals themselves need not represent densities directly and need not bealgebraically added. For example, they can represent the transmittancevalues of transparencies corresponding to the two negatives. In thiscase, since the transmittance values vary inversely with theantilogarithms of the density values, one signal must be divided by theother in order to obtain the effect of adding the densities in opposingsense.

In the preferred embodiment of the invention the correcting signal isused to modulate a light source the light from which passes through anuncorrected transparency, representing the color to be corrected on itsway to the photographic emulsion which is to be exposed.

cathode raytube, on the face of which a scanning raster is formed, themodulation of the electron beam by the correcting signal causing theimage on the face of the cathode ray tube to constitute a correctinglight mask.

A number of separate scanning beams from the modulate'd light source maybe transmitted through a number of uncorrected transparencies tolight-sensitive devices which provide electric signals representingquantities of light which vary in accordance with the modulated sourceand with the transmittance values of the transparencies, and theemulsion to be exposed may be placed behind the uncorrected transparencyfor which the correction has been computed, the light passing throughthe latter and through the said emulsion to the associatedlight-sensitive device. i 'It is preferably arranged that the signalswhich are used to form the correcting signal are combined at an earlystage in such a manner that signals corresponding to neutral tones inthe original are substantially cancelled out. Any desired form ofnon-linearity can be introduced after this stage, to correct the colorvalues, without affecting the neutral tones. I

Good results can be obtained when the non-linear circuit includes as thenon-linear element a component having a sharp cut-01f, for example adiode.

In order that the invention may be better understood two embodimentsthereof will now be described with reference to the accompanyingdrawings, in which:

FIGURE 1 is an explanatory diagram;

FIGURE 2 is a schematic diagram of a system according to the presentinvention;

FIGURE 3 shows an alternative form of correction computing unit; and

FIGURES 4, 5, 6 and 7 are circuit diagrams giving details of circuitsused in the apparatus of FIGURES 2 and 3.

The method according to the invention will now be described withreference to FIGURE 1. For the purpose of this explanation, it will beassumed that a photographic plate having exposed upon it three squaresof differing density is used to etch a cylinder to print cyan densitydistribution is represented by diagram (a) in FIG- URE 1, in which theordinates represent densities across the plate. This diagram shows thateach square is of uniform density but that the densities of the squaresincrease from left to right. Another plate exposed to the same threedensity levels but having in each square the step-wedge densitydistribution indicated in diagram (b) (i.e. three strips of increasingdensity) is used to etch a cylinder which is to print magenta Theconcentration of the inks is balanced so that when the cyan, magenta andyellow inks are superimposed on White paper, equal amounts of the inkswill produce a grey tone or black. The superimposition of equal amountsof cyan 55 and magenta produces the neutral color, blue.

The two cylinders are used to make a print in which cyan and magentainks are superimposed. This colored subject is then photographed throughred and green filters respectively, and contact positives are made fromthe two 80 negatives. If the process described so far has been linearand if the inks are free from impurities, then these positives would bethe same as the original positives (a) and (b). It is found, however,that whilst the new cyan positive agrees fairly well with the originalcyan plate rep-re- 66 to say in the first step of the first square, thesecond 70 step of the second square, and the third step of the thirdsquare. This is because the concentration of the inks has been balancedto provide this agreement for the neutral tones. Because of additivityfailure, however, the

" aa mrl A i a creased, the decrease in contrast being greatest in theright-' hand square in which a large amount of cyan has been printed.Diagram (0) also represents the signal in logarithmic form obtained froma photomultiplier during scanning of the corresponding magenta negativewith an unmodulated light source. If the light source could be modulatedinsuch a manner that the magenta signal from the photomultiplierappeared as in (b) instead of as in (c), then this signal could be saidto represent a corrected magenta positive. A plate exposed through themagenta negative by means of a light source which is modulated in thismanner would form a corrected magenta positive. If this were now used inconjunction with the cyan positive,'a second print would be obtainedwhich, when photographed through red and green filters, would providenegatives such that the corresponding positives would be close copies ofthose represented at (a) and (b). a

To obtain the signal which is required to modulate the light source, asignal corresponding to the new cyan positive (i.e. correspondingapproximately to diagram (a)) is inverted (see diagram (d)) and combinedwith a signal corresponding to the new magenta-positive (diagram (0) Theresultant signal is as shown at (e).

The waveform (e) is passed through a non-linear circuit which isarranged to attenuate this signal when it exceeds a given value (i.e.when the magenta signal (c) exceeds the cyan signal). The e'ifect of acircuit of this kind is seen in the waveform (f). This waveform is thecorrecting signal and is used to modulate the brightness of a scanninglight source which is used to transmit light through the uncorrectednegative (corresponding to wave form (0)) to the emulsion to be exposed.The waveform (1) therefore also represents the spot brightness level forthe scanning of a single line of the uncorrected magenta negative. Thebrightness level of the light reaching the emulsion to be exposed (afterpassing through the uncorrected negative) then takes the form shown at(g), which has been obtained by combining diagram (1) with theun'corrected'magenta signal in diagram (0). It will be see'nthat themagenta contrast (the difference in amplitudebetween the first and thirdstrips of the square) is restored in the second and third squares,'andthat diagram '(g) closely resembles diagram (b). The condition hastherefore been fulfilled, showing that Waveform (f) is the requiredcorrecting waveform.

It is seen that the signals cancel for the densitiesreprcsenting neutralcolors and no modulation occurs in these areas.

The apparatus which is used to carry out the method according to theinvention will now be described.

In FIGURE 2 the cathode ray tube 10 constitutes the light source forscanning the uncorrected negatives and also for exposing the emulsionwhich is to form the corrected printer positive forone of the printercolors. The light spot on the face of the cathode ray tube which ismodulated in accordance with the required correction for one of thecolors, is focused through three lenses 12 onto three uncorrectedseparation negatives 14 so that the three negatives are scannedsimultaneously and in synchronisrn. The light beams pass through thenegatives onto photomultipliers 16. Assuming that the middle negativecorresponds to the color for which a corrected printer negative is beingprepared, the emulsion which is to become the corrected printer positiveis placed immediately behind the middle negative 14, as shown at 18.Thus the scanning light spot reaching the emulsion 18 containsvariations due to the modulation of the brightness of the cathode raytube spot and also variations to the uncorrected negative '14. Theemulsion 18 may be blue-sensitive so that, if it is provided with theyellow backing 20, the yellow component of the light will pass throughto the photomultiplier 16 but the blue component will be prevented fromfogging the plate by reflection contrast between the densities in eachsquarev has .deto the back of the 61111 519 1.-

The light passing through the middle negative 14 is diffused by theemulsion 18 and in order to diffuse the transmitted light in the upperand lower channels in a similar manner, diffusing plates 22 are placedbehind the upper and lower negatives 14. The output signals from thephotomultipliers 16 are applied to logarithmic circuits 24 which providesignals representing the logarithms of the input signals. The signalsfrom the logarithmic circuits are applied directly to contacts a ofswitches 26 and also to inverting circuits 28, as a result of which thelogarithmic signals are applied in inverted form to contacts 0 of theswitches 26; contacts b are not connected. The switch 26 may thus beused to select any or all of the three signals in either polarity. Themovable contacts of the switches 26 are connected to a summing amplifier30 in which, in the example shown, a positive signal from the middlechannel is added to a negative signal from the upper channel, the lowerchannel being disconnected. The output of the summing amplifier 30 isapplied to a mixer 32 which receives a driving voltage from a drivecircuit 33, this driving voltage determining the reference level of thebrightness of the spot on the cathode ray tube. The signal from themixer 32 is then applied to an antilogarithmic circuit 34 by means ofwhich signals are derived representing the antilogarithms of the signalsfrom the mixer. These antilogarithmic signals are applied to anon-linear circuit 36 by means of which they are attenuated when thedifference between the density of the color to be corrected and thedensity of the correcting color exceeds a predetermined amount (seeFIGURE 1(f)). The output of the non-linear circuit 36 constitutes thecorrecting signal and is applied to the grid of the cathode ray tube.

If the upper channel in FIGURE 2 is the cyan channel and the middlechannel the magenta channel, the signals from the upper photomultiplier16 will be proportional to BXC, where B is the spot brightness and Crepresents the transmittance of the scanned portion of the cyan negativeat any instant. Similarly the signals from the photomultipler 16 in themiddle channel will be proportional to BXM where M represents thetransmittance of the magenta negative at some point. After passingthrough the logarithmic circuits 24, the signals represent log B+log C,and log B+log M respectively. Thus the signals which are applied to thesumming amplifier 30 in FIGURE 2 are (log B+log M) and (log B+log C). Itwill be seen that the result of this is log M -log C, the brightnessfactor having cancelled out. The values log M and log C represent thedensities of the cyan and magenta negatives respectively, and it is thedensity difference signal which is applied to the antilogarithmiccircuit 34, the output of which represents the quotient M/ C of thetransmittance values of the two negatives. The output signal from thenon-linear circuit modulates the brightness of the spot on the face ofthe cathode ray tube in accordance with the expression and thetransmittance of the uncorrected magenta negative, and is given by M f(%K) or in terms of the densities of the colors log M+f(log M-log C+log K)It will usually be required that the neutral tones, that is to say thosewith equal cyan and magenta contents, shall be reproduced withoutchange, and this is easily .arranged by making the signal log M log C ofreference level when cyan and magenta tones of equal densities are beingscanned. If desired, however, a tone compensation circuit may beincorporated for the purpose of modifying the reproduction of neutraltones.

If the yellow signal in the lower channel is also used to correct themagenta signal, the lower part of the switch 26 is set to contact 0, sothat the output of the adding circuit 30 represents log M log C-log Y,where Y is thetransmittance value of the yellow negative. In this case,however, the reference level of the output signal corresponds to thevalue obtained when M, C and Y are all of equal value.

In the example described above the factor representing the modulation ofthe brightness of the spot on the face of the cathode ray tube wasremoved during the summation in the amplifier 30. If for any reason itis required to remove this factor before the summation of the signals,each signal can be separately demodulated in the following manner. Aphotomultiplier 38, shown in dotted lines in FIGURE 2, is exposeddirectly to the face of the cathode ray tube and its output signal isapplied to a logarithmic circuit 49 and then through an invertingcircuit 42. The output of the inverting circuit is then algebraicallyadded to the output signals from each of the logarithmic circuits 24. Itwill be seen that the subtraction of the logarithm of the signal fromphotomultiplier 38 from each of the signals in the colour channels(representing the logarithms of the outputs of photomultipliers 16), isequivalent to dividing the signals from the photomultipliers 16 by afactor representing the brightness of the tube at any instant. Thus thethree resultant signals will represent the logarithms of thetransmittance values (that is to say the densities) of the threenegatives.

With some emulsions, it has been found possible to emit the filterbacking 20 and therefore to extend the plate sensitivity into the greenor even into the red part of the spectrum.

FIGURE 3 shows a simpler embodiment of the invention in which the outputsignals from the photomultipliers 16 are applied directly to thecontacts a of the switch 26, and through the inverting circuits 28 tothe contacts c. The signals selected by means of the movable contacts ofthe switch 26 are applied to the summing amplifier 30, the resultantpassing to a non-linear circuit 36 and thence to the mixer circuit 32where it is combined with the drive signal from the circuit 34. Althoughin this case the transmittance values are subtracted directly, withoutcouverting them to density values by taking logarithms, it is found thatgood results are obtained if the signals are maintained within asuitable range of values. The subtraction cancels to some exent themodulation present in the colour signals due to the modulation of thelight spot on the face of the cathode ray tube. Again if completedemodulation is required the output of the photomultipliers can beconvertedto logarithmic form and a signal derived from thephotomultiplier 38 (FIGURE 2 also converted to logarithmic form, can besubtracted from the colour density signals.

Although it is preferred to efi ect multiplication of the transmittancesignals by logarithmic conversion followed by summation, it is alsopossible to employ multigrid valves in known manner to effect themultiplication directly.

FIGURE 4 shows a circuit by means of which the signal from thephotomultiplier 16 may be made to bear a logarithmic relationship to thecurrent flowing to the collector of the photomultiplier, and thereforeto variations in the light falling upon the photomultplier. Thecollector of the photomultiplier is connected to the H.T. positiveterminal by way of a load resistance- 44 which takes the form of agermanium or silicon diode. Since the photomultiplier itself has a veryhigh impedance, reasonable variations of the load resistance 44 haveonly a very small effect on the photomultiplier current. As theresistance of the germanium or silicon diode 46 decreases exponentiallyas the current which flows through it increases over the range ofcurrent required, the voltage across the diode follow a logaiithmiccprve when plotted against variations in the light falling on thephotomultiplier.

1 FIGURE 5 shows the basic form of an antilogarithniic circuit employinga germanium or silicon diode 46. The input voltage is applied across thediode 4a in series with a resistor 48 which is of small value comparedwith the resistance of the diode, and the output is taken across theresistor 48. If an appropriate range of current and voltage'ismaintained, the resistance of the diode will decrease exponentially withincrease of current, and the voltage across the linear resistor 48 willbe proportional to the current flowing through the combination and willtherefore represent the antilogarithm of theinput voltage. This is onlytrue, however, if the resistance of the component 48 is very smallcompared with that of the diode 46, in which case the signal across theresistor 48 is very small. Figure 6 shows a circuit in which the inputto the diode is obtained from a cathode follower 50 and is appliedacross the diode 46, a resistor 52 and a resistor 54 in series. Theresistor 54 is the cathode resistor of a triode 56, and a signal derivedfrom the junction of the resistor 52 and the diode 46 is applied througha triode 58 to the grid of the triode 56. The resistance across thecomponents 52 and 54 appears very small [8.5 a result of the negativefeedback which is aplied across the resistor 54in this manner. Theamplified output signal is obtained from the anode of the feedbackamplifier triode 58.

In FIGURE 7 there is shown an example of a simple non-linear amplifier."in this circuit the input signal is applied to the grid of a triode 6%,the anode of which is connected through a non-linear component 62 to thejunction of two resistors 64 and 66 which are connected in series acrossthe HI. supply. The output is taken from the junction of these tworesistors. The component 62 may be a diode with a sharp cut-oil, sodirected that current will pass from left to right in FIGURE 7 but notfrom right to left. In this way information is passed to a subsequentstage when the signals are such that the anode of the triode 60 ispositive with respect to the junction of the resistors 64 and 66, butwhen the anode is negative with respect to the potential of thisjunction, the voltage of the output conductor is maintained at thejunction potential.

Instead of having a sharp cut-ofi the non-linear circuit may have, forexample, an exponential slope.

In some cases, to simplify the design of amplifying circuits it may bedesirable to modulate the light beam at constant amplitude and highfrequency (in addition to the modulation by the correction signal) sothat the output of the photomultipliers is an alternating voltageresembling a modulated carrier wave.

The inverter circuits may be single triode circuits to the grid of whichthe input signal is applied, the inverted output signal being obtainedfrom the anode of the triode. The summing amplifiers may be of the knownkind in which the signals to be summed are applied through individualresistors to the grid of a summing triode, the voltages across the gridresistor of the triode being kept small by the introduction of negativefeedback from the anode to the grid through a resistor of suitablevalue.

In the circuits described above the correction signal has been used toform a light mask on the face of the cathode ray tube which exposes anemulsion through the negative to be corrected, and the same cathode raytube has been used to scan the uncorrected'negatives to provideinformation from which the correcting signal is derived. It is not,however, essential for the same light source to be used both to obtainthe information and to expose the printer emulsion. Furthermore insteadof using the correcting signal to form a light mask on the face of thetube, the correcting signal can be multiplied by signals representingthe transmittance of the uncorrected negative for the printer inquestion (or the logarithms of these signals may be added) to provide aresultant corrected signal which can be used to modulate 8 alight sourcewhich exposes the printer emulsion d1 rectly,'

Satisfactory reproduction of the grey scale is a very important part ofthe overall reproduction process, and

is e ss ential that a system for producing photographic records fromwhich suitable prints can be mad'e'should not only'provide adequatecolour correction, but also be.

capable of a reproduction of the grey scale which is eithersubstantially perfect or is modified in a controlled manner. In themethoddescribed above, freedom from distortion is achieved by setting upthe scanner in such a manner that the grey scale cancels out in themask," so that the grey scale appearing in the corrected positive is adirect reproduction of that in the negative. This is done by arrangingthat, for grey tones, the amplitudes of the two signals used to form themask cancel out in the subtraction stage. If, however, it is required toproduce a positive in which the grey scale is difierent from that in thenegative, a tone compensating circuit can be incorporated.

The scanner which has been described may be used to carry out theelectronic equivalent of a photographic technique which may be calledsuccessive two-stage masking. Considering the masking of the yellowprinter by the magenta printer, in the photographic process a positivemade from the uncorrected yellow negative is registered with theuncorrected magenta negative, both being made to a gamma of unity sothat the grey tones cancel out. :From this combination a mask is made,and this mask is used with the uncorrected yellow negative to. exposethe first corrected yellow positive. This is usually the end product oftwo-stage masking. If, however, this first corrected yellow positive isregistered with the uncorrected magenta negative and another mask madefrom the combination, this second mask can be registered with theuncorrected yellow negative to mask a second corrected yellow positive.This process can go on indefinitely, the amount of correction carriedout becoming progressively less at each stage. It will be seen that theprocess soon approaches a point where the mask is made 'from thecombination of a corrected yellow value and an uncorrected magentavalue. This kind of result can be easily achieved in the scannerdescribed above by using its feedback properties, and the resultingmethod is an important form of the present invention.

In one form of this method, the signals from the yellow and magentachannel photomultipliers are passed through logarithmic circuits and thesignal in the magenta channel is demodulated to remove the elfect ofvariations in spot brightness. The resulting signal in the magentachannel, representing density values of the uncorrected magentanegative, and the signal in the yellow channel, representing densityvalues of the corrected yellow negative, are fed in phase opposition toa summing amplifier, the output of which is fed through anantilogarithmic and a limiting circuit to the grid of the cathode raytube.

The use of a scanner has, of course, considerable advantages in thesaving in photographic materials and reduction in processing time. Theuse of a single light source for both analysis and reproduction, as inthe system described above, has further advantages in that the problemsof accurate registration are largely overcome, as are other problemsassociated with possible differences arising from separate analysing andreproducing light sources.

We claim:

1. Color reproduction apparatus comprising an electro-optical scanner, acorrection path including a circuit in which at least one electricsignal from said scanner representing a correcting color is subtractedfrom an electric signal representing a color to be corrected, anon-linear circuit for receiving the output signal from said subtractioncircuit and having an input-output characteristic in which the gain isreduced as the input increases thereby providing an uncompensatednon-linearity in said correction path, and a light source the brightnessof which varies with the output of said non-linear circuit and which isarranged to expose a photographic emulsion which will provide theprinter for the given color.

2. Apparatus according to claim 1, in which said light source isarranged to expose said photographic emulsion through an uncorrectedtransparency representing the color to be corrected.

3. Apparatus according to claim 1, including a circuit in which thesignal from said non-linear circuit is combined with an uncorrectedelectric signal representing the given color to be corrected, to providea resulting corrected signal for the given color which is used tomodulate said light source, whereby the brightness of said light sourcevaries both with the uncorrected signal and with the corrected signalfrom the non-linear circuit.

4. Apparatus according to claim 1, in which the nonlinear circuitcontains as its non-linear element a diode.

5. Apparatus according to claim 1, in which the light source is acathode ray tube.

6. Apparatus according to claim 1, in which said scanner includessignal-generating means which supply to the subtracting circuit signalsrepresenting the density of the color to be corrected and the density ofat least one correcting color.

7. Apparatus according to claim 1, in which the density signals fromsaid signal-generating means each include signal variations representinga correction to be applied to a given color, whereby said signalvariations cancel out in the subtraction process.

8. Apparatus according to claim 1, in which said scanner includessignal-generating means which supply to said subtracting circuit signalsrepresenting the transmittance of the color to be corrected and thetransmittance of at least one correcting color.

9. Apparatus according to claim 8, in which the transmittance signalsfrom said signal-generating means each include signal variationsrepresenting a correction to be applied to a given color whereby saidsignal variations cancel out in the subtraction process.

10. Apparatus according to claim 1, in which said non-linear circuitprovides an output which varies with its input when the signalrepresenting the correcting color exceeds the signal representing thecolor to be corrected but is of constant level when the signalrepresenting the color to be corrected exceeds the signal representingthe correcting color.

11. Apparatus according to claim 1, in which said nonlinear circuitprovides an output which varies with its input when the sum of thesignals representing the correcting colors exceeds the signalrepresenting the color to be corrected but is of constant level when thesignal representing the color to be corrected exceeds the sum of thesignals representing the correcting colors.

12. Color reproduction apparatus comprising an electro-optical scanner,a correction path including a circuit in which an electric signal fromthe scanner representing the transmittance of a transparencycorresponding to a color to be corrected is divided by at least oneelectric signal from the scanner representing the transmittance of atransparency corresponding to a correcting color, the apparatus furthercomprising a non-linear circuit which receives the output signal fromsaid dividing circuit and having an input-output characteristic in whichthe gain is reduced as the input increases thereby pro- 10 viding anuncompensated non-linearity in said correction path, and a light sourcethe brightness of which varies with the output of said non-linearcircuit and which is arranged to expose a photographic emulsion whichwill provide the printer for the given color.

13. Color-correction apparatus comprising an electrooptical scanner forscanning an original or color separations therefrom, a correction pathincluding a circuit for combining an electric signal from said scannerrepresenting a color to be corrected with at least one electric signalfrom said scanner representing a correcting color so that a function ofthe density of the correcting color is subtracted from a function of thedensity of the color to be corrected, a non-linear circuit for receivingthe output signal from said combining circuit and having an input-outputcharacteristic in which the gain is reduced as the input increasesthereby providing an uncompensated non-linearity in said correctionpath, a light source modulated with the output of said non-linearcircuit to vary its brightness, said light source, which forms part ofthe electro-optical scanner, scanning said original or color separationsand also scanning, through a color separation transparency to becorrected, a photographic emulsion which will provide a printer for thecolor which is being corrected.

14. Apparatus according to claim 13, in which the color-representingsignals which are combined are so adjusted that the resultant signalrepresents zero correction when the tone represented by the uncorrectedsignals is a neutral tone.

15. Apparatus according to claim 13 including beamsplitting meanswhereby separate scanning beams from the light source are transmittedthrough a number of uncorrected transparencies, a number oflight-sensitive cells arranged to receive the light beams passingthrough the transparencies and providing corresponding electric signals,and means for accommodating the photographic emulsion to be exposedbehind and substantially in contact with the uncorrected transparencyfor which the correction has been computed so that light passing throughthe latter to the associated light-sensitive device also passes throughsaid emulsion to be exposed.

16. Color reproduction apparatus comprising an electro-optical scanner,a correction path including a circuit in which an electric signal fromsaid scanner representing the transmittance of a transparencycorresponding to a color to be corrected is divided by at least oneelectric signal from said scanner representing the transmittance of atransparency corresponding to a correcting color, a non-linear circuitfor receiving the output signal from said dividing circuit and having aninput-output characteristic in which the gain is reduced as the inputincreases thereby providing an uncompensated nonlinearity in saidcorrection path, and a light source the brightness of which varies withthe output of said nonlinear circuit, for exposing a photographicemulsion which will provide the printer for the color which is beingcorrected.

References Cited in the file of this patent UNITED STATES PATENTS2,710,889 Tobias June 14, 1955 2,721,892 Yule Oct. 25, 1955 2,757,571Loughren Aug. 7, 1956

